Amhed,

• If applying % to total duration it is to be expected to get some fractional days as remaining duration. Say 50% duration on a total of 3 days equal 1.5 days.
• Instead of using %duration you can use whole days for remaining duration, but this will have the effect of delaying some unwanted activities.
• No matter what, in practice some activities will be delayed on the field for reasons such as it does not make sense to start the activity if it is not going to be finished the same day, another reason will be after certain hour it does not make sense to start certain activity. On the other hand for many activities it makes perfect sense to start as soon as possible even if it is not at first hour of the work day.
• This is one of the many reasons why we should schedule considering management of buffer time.
• I favor using hours as the planning unit and not be so concerned about the precise hour the activity is scheduled to start because you never know ahead of time the exact hour, better manage on the optimistic schedule than a delayed schedule. 
• If required to round remaining durations I would round up greater than 1 day remaining durations to the current integer if fractional is </= 0.75 to the next integer if not, instead of using 0.50 as the threshold. If remaining duration is less than 0.75 days 0 is not an option but fraction of a day might.
• I do not believe there are simple rules except not rounding at all.
• Best Regards,

Rafael

From the PMI Practice Standard for Scheduling.

Article 3.3.2.5
Float values are calculated by comparing the early and late dates as follows:
-Total float is calculated by subtracting the early finish date from the late finish date (start dates can be used instead).
-Free float is calculated by subtracting the early finish date of the activity from the earliest start date of the closest of its successors. Free float is never negative.

Article 4.2
Definition for Total Float:
Represents the amount of time an activity can delay its finish without impacting the late finish of the project. It is computed as the difference between the late and early dates of the activity, calculated from the forward and backward passes respectively. As

Definition for Free Float:
Represents the amount of time an activity can delay its early finish without impacting any successor activities early start. It is the difference between an activity’s early finish date and the earliest start date of the closest of its successors.

Just burn the PMI document as it is inconsistent, at times it defines float using start dates while at times it defines float using finish dates. This is not necessarily true after progress is reported or when using non continuous PDM (split activities).

What blow out our mind is that some PMI members are proponents of the Non Continuous PDM Method, a method that allows for automatic activity spitting, this is wrong in the case you want to keep the activity continuous, this hides that individual segments do have early and late dates and floats of their own. This is also mostly implemented across the board and not at the activity level, in both cases a wrong approach, shall not be automated. It is geared to please the lazy scheduler. Simply separate activities into two or more individual activities and allow the software to display all values.

Look at P3, it allows you to make it your own definition, either the difference of start dates or the difference of finish dates while Microsoft Project plays the roulette by allowing it to be the largest of the two. Well to kick the activity laying down on the floor some software allows the use of resources to be discontinuous along the activity duration, this fools the resource leveling algorithm and again is geared to please the lazy scheduler. Resource usage also have float, let the software math see it, create separate activities when needed to properly model your job.

Best regards,
Rafael

Hi Ahmed,

The physical meaning is that how much "spare time" you have before you start delaying the activity and the overall project.

Slack or float is good because it takes care of the uncertainities and the management reserve for it.

The Schedule is as good as your plan. Physically, you have to assess (this is a repetitive task that you can do on a daily basis) how the delay of the ongoing activities will affect the overall project duration.

Question: what is the size of your project please.

With kind regards,

Samer